This invention relates to papermaking and more particularly to an apparatus and a method for controlling the temperature of a web or sheet of paper as it enters the press section through the utilization of a stepping motor to open and close valve pipes in order to control the application of steam against the sheet surface and the corresponding withdrawal of the steam by a suction device through the opposite side of the sheet that contacts the forming fabric (wire).
In the forming section of a paper making machine, water is removed from the pulp to form a web. The drainage rate of the water is proportional to the viscosity and surface tension of the trapped water. Further water expression (removal) is accomplished in the press section. Increasing sheet temperature decreases the water viscosity and surface tension, hence augmenting the expression of water. As shown in U.S. Pat. Nos. 3,574,338, 3,945,570, 4,050,630 and 4,163,688, it is common practice to apply steam to a sheet in the forming section and prior to the sheet entering the press section so that the latent heat of the steam increases the temperature of the sheet.
It is a common practice to utilize a vacuum source beneath the web to remove water. The vacuum source can also serve to draw the steam into the web and increase steam penetration. This increased steam penetration serves to increase the operating efficiency of the steam shower and the heat transfer to the web. This vacuum source is typically run at a constant uniform vacuum level in the transverse or cross direction (CD) without any regulation of volume once the machine has stabilized.
The press section of a paper making machine is located before the dryer section. Therefore, increasing the water removal rate through the press section serves to decrease the moisture content of a sheet entering the dryer section, thereby either reducing the energy consumption required to further dry the sheet or increasing production (speed) at constant dryer section energy consumption.
Typically when steam is applied to a section of the web or sheet with excess moisture, the steam will migrate to an alternate section of the web or sheet where there is less moisture before it penetrates the web. This happens because where there is less moisture the opposing vacuum system works "better" and actually more suction gets through the web to withdraw the steam. This tends to exaggerate rather than solve the moisture problem. This invention se&s out to eliminate this "steam migration" problem by applying vacuum to the appropriate section effectively preventing the steam from travelling to a "dry spot" in the web. Water expression is also proportional to the level of vacuum applied. Thus applying higher vacuum to that portion of the web with higher moisture increases the water removal rate.
In any steam application, consumed steam should be maximized for its effective use. To maximize the effective usage, the percentage of consumed steam that condenses on and in the sheet for the purpose of raising the sheet temperature should be maximized, and the percentage of consumed steam that does not condense but instead exhausts to the atmosphere as wasted energy should be minimized.
There are certain applications where the steam application does not have to be positionally and volumetrically controlled. In other applications however, it is necessary to impart steam to the process in controlled amounts at specified positions across the machine for profiling certain sheet qualities. This controlled imparting of steam is commonly performed as part of a closed-loop control system, where the sheet quality variable in question is scanned on-line at equally spaced increments across the machine. The results obtained by the scanning device, through the use of computer analysis, are used to automatically control the steam flow applied to the sheet in accordance with the desired sheet quality criteria.
The ability of known steam shower apparatus to repeatedly apply a uniform steam flow is presently limited to the accuracy and repeatability of pneumatically actuated control valves.
For the same reasons that it is important to accurately control the steam flow to the sheet, it is also important to maintain uniform heat-transfer, over the portion of the sheet in question.